Übergangsmetallaktivierte organische Verbindungen, XXXVIII. Chemoselektive nucleophile Methylierungen durch In‐Situ‐Blockierung von Aldehydgruppen unter α1‐Phosphonioalkoxid‐Bildung

Transition Metal‐Activated Organic Compounds, XXXVIII[1]. — Chemoselective Nucleophilic Methylation Reactions by In‐Situ‐Blocking of Aldehyde Groups by α1‐Phosphonioalkoxide Formation Herrn Professor Ulrich Schöllkopf zum 65. Geburtstag gewidmet. For the selective methylation of a keto group in the presence of an aldehyde group the complex MeTiCl3·PPh3 and especially the reagent system [TiC]4·PPh3 + 0.5 Me2Zn] proved to be favourable. For instance, 6‐oxoheptanal (8) was methylated by [TiCl4·PPh3 + 0.5 Me2Zn] at the keto group with 89% yield and 99:1 selectivity, whereas it was methylated by [TiCl4 + 0.5 Me2Zn] at the aldehyde group with 81% yield and 96:4 selectivity. A selective methylation of benzaldehyde in the presence of heptanal was achieved with MeNbCl4·PPh3 or [NbCl5·PPh3 + 0.75 Me2Zn] to give (1‐chloroethyl)benzene (14; yield 92 or 53%; selectivity in each case 93:7) or with [TaCl5·PPh3 + 1.5 Me2Zn] to give mainly 1‐phenylethanol (15) besides 16. A 96:4‐selective benzylation (yield 74%) of a ketone in the presence of an aliphatic aldehyde was possible with [TiCl4·PPh3 + 1 PhCH2MgBr]. — The high chemoselectivity is caused by irreversible blocking of aliphatic aldehyde groups by α1‐phosphonioalkoxide formation[1], whilst the corresponding reaction of keto groups[1] and of benzaldehyde is reversible.